Explosive shock dissipater

ABSTRACT

A method of dampening stress waves propagated from the detonation of an explosive device in a wellbore to protect a device positioned downhole including the steps of positioning a shock dissipater between an explosive device and a package; detonating the explosive device propagating a stress wave toward the shock dissipater and the package; and reflecting a portion of the stress wave in the shock dissipater. The shock dissipater may have one or more interface formed between materials of dissimilar acoustic impedances. The shock dissipater may reflect a compressive portion of the stress wave at one interface and a tensile portion of the stress wave at another interface.

TECHNICAL FIELD

The present invention relates in general to explosive devices and more particular to devices and systems for shielding devices from the stress energy propagated from explosions by dissipating the propagated stress energy.

BACKGROUND

It is often necessary to use explosives during wellbore operations, in particular during the drilling of a well. For example, it may be desired to obtain a sidewall core before the well is completed. To obtain a core, an explosive charge may be detonated to propel a coring device into the wall to obtain a sample. It is also commonly required to provide communication between the wellbore and the surrounding formation after the well is completed with a casing. To provide the necessary communication, perforations are created through the casing, cement and into the formation. The perforations are commonly created with shaped explosive charges.

These explosive devices may be lowered and positioned at the desired location in various manners. In the simplest of operations the explosive charge may be positioned by measuring the length of the conveyance, carrying the explosive charge, that is run into the well until it is believed that the explosive charges are properly positioned.

In many instances it is desired or necessary to include electronic instrumentation or other mechanical packages in close proximity to the explosive charges or other working packages that may be sensitive to the explosive shock waves. For example, in high-angle wells and horizontal wells it may be necessary to include locating electronics with the conveyance and explosive device for proper positioning. Electronic equipment and the like are also desirable for locating thin zones and positioning of the explosive device. Further, in deep wells the stretch of conveyance (wireline or pipe) alone may necessitate the use of locating equipment for positioning. Additionally, it is often desired to perform multiple operations in a well without running into and then out of the well between operations, in these cases it is desirable to include electronic equipment in the tool string.

The firing of the explosive charges produces a large shock wave that propagates through the wellbore and perforating assembly. These shock waves can exert massive forces upon the electronic instrumentation and other downhole equipment, often resulting in damage to or failure of the equipment. It may be desired to shield any number of various devices, or packages, from the shock waves propagated from the explosive device. Packages to be shield from the explosive shock waves may include, without limitation to, sensors, gauges, logging instruments, telemetry devices, additional explosives, and valves.

SUMMARY

Examples of apparatus, devices, systems and method for dissipating a stress wave to shield a package such as, without limitation, an electronic system or working member from the stress wave. In one example, an apparatus for dampening a stress wave includes a first interface formed between a first layer of material and a second layer of material, the first and second layers of material having dissimilar acoustic impedances.

In another example, a wellbore tool string includes an explosive device, a package, and a shock dissipater positioned between the explosive device and the package, the shock dissipater includes a first interface formed between a first layer of material and a second layer of material, the first and second layers of material having dissimilar acoustic impedances. Additional layer of alternating acoustic impedance material can be add to further reduce the shock.

In another example, a method of dampening stress waves propagated from the detonation of an explosive device in a wellbore to protect a device positioned downhole includes the steps of positioning a shock dissipater between an explosive device and a package; detonating the explosive device propagating a stress wave toward the shock dissipater and the package; and reflecting a portion of the stress wave in the shock dissipater back to the originating device and propagating a smaller portion of the shock wave through the device.

The foregoing has outlined some of the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and aspects of the present invention will be best understood with reference to the following detailed description of a specific embodiment of the invention, when read in conjunction with the accompanying drawings, wherein:

FIG. 1 is a wellbore schematic illustrating an example of a tool string incorporating a shock dissipating system of the present invention;

FIG. 2 is partial cross-sectional view of an example of a shock dissipater incorporated into a sub assembly; and

FIG. 3 is an illustration of a shock dissipater in isolation.

DETAILED DESCRIPTION

Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views.

As used herein, the terms “up” and “down”; “upper” and “lower”; and other like terms indicating relative positions to a given point or element are utilized to more clearly describe some elements of the embodiments of the invention. Commonly, these terms relate to a reference point at the surface from which drilling operations are initiated as being the top point and the total depth of the well being the lowest point.

FIG. 1 is a schematic illustration of an example of a system for dissipating the shock from an explosion, generally denoted by the numeral 10. System 10 includes a tool string, generally denoted by the numeral 12, suspended by a conveyance 14 in a well or wellbore 16. Conveyance 14 may include without limitation wireline, slickline, coiled tubing, jointed tubing and the like for carrying and suspending tool string 12 in wellbore 16. It is recognized that conveyance 14 may be included in whole or in part with tool string 12.

In the illustrated example, tool string 12 includes an explosive device 18, a shock dissipater 20, and a package 22. Explosive device 18 is illustrated as a perforating gun; however explosive device 18 may be without limitation a coring device, a cutting tool or other device that includes an explosive charge or other element that produces a shock wave upon activation.

Package 22 may be any device, tool or system that it is desired to be protected from the shock waves propagated from the detonation or activation of explosive device 18. For purposes of description herein, package 22 is often referred to generally as an electronics package. For example, package 22 may include electronics and instrumentation for sensing pressure or temperature. Package 22 may be an electronics package such as, without limitation, logging instruments, telemetry equipment, and positioning instruments. However, it is recognized that package 22 may include any other device that may need to be shielded from the shock waves of the explosive device, including without limitation, valves and explosive devices.

Shock dissipater 20 is provided for dampening shock energy generated by downhole explosive device 18, thus providing a means for running packages 22 in tool string 12. In FIG. 1, dissipation device 20 is illustrated as a sub assembly that is positioned within tool string 12 between perforating gun 18 and package 22. However, in other examples dissipating device 20 may be formed for example as part of package 22 or within explosive device 18.

Refer now to FIG. 2 wherein an example of shock dissipater 20 is illustrated. Dissipater 20 of the present example is illustrated incorporated into an assembly that is adapted for threadably connecting within tool string. Dissipater 20 includes a body or housing 24, which in the illustrated example is a tubular collar having a longitudinal axis 26. In this example, housing 24 forms an axial bore 28 that, may or may not, extend the full length of housing 24, as will be further explained below.

Dissipater 20, which will be described in more detail below with reference to FIG. 3, includes two dissimilar members 30 a and 30 b that form an interface 32 therebetween. Dissipater 20 is connected within housing 24 so that interface 32 is positioned approximately perpendicular to longitudinal axis 26. A conduit 34, indicated by the dashed lines, may be formed through shock dissipater 20. In this example, conduit 34 is oriented substantially parallel to longitudinal axis 26 and bore 28. Conduit 34 may be provided, for example, for passing wiring (not shown) to perforating gun 18 (FIG. 1) or other electronics, or for passing a drop bar for detonating explosive device 18.

Referring now to FIG. 3, wherein an example of shock dissipater 20 is shown in isolation. In the illustrated example, dissipater 20 is constructed of three members or layers denoted by the numeral 30 a, 30 b, and 30 c. Although the Figures only illustrated examples having either two layers 30, one interface 32, or three layers 30, two interfaces 32, more than three layers 30 may be utilized. Each layer 30 is constructed of a material having acoustic impedance (I), which is defined generally as the acoustic sound speed (c) in the material multiplied by the density (ρ) of the material of construction.

Layers 30 are positioned in contact with one another to form interfaces 32. Adjacent layers 30 have dissimilar acoustic impedance, and are referred to herein as dissimilar layer or members. Thus, layer 30 a has an impedance I_(a) and layer 30 b has an impedance of I_(b). Similarly, 30 c has an impedance I_(c), which is different from and therefore dissimilar to its adjacent layer 30 b. Layer 30 c has an impedance that is dissimilar to layer 30 b, and that may be the same as or different from the impedance of layer 30 a.

Upon detonation of explosive device 18, shock or stress waves, illustrated by arrows 36, propagate through the first dissipating layer 30 a encountered. Stress waves 36 include a tensile portion and a compressive portion. When shock waves 36 encounter interface 32 ab, a portion of the stress energy is transmitted into the adjacent layer 30 b, illustrated by the arrow 36 t, and a portion is reflected, illustrated by the arrow 36 r. When stress energy 36 t encounters interface 36 bc a portion is transmitted and a portion is reflected. Any of the interfaces could be angled so as to deflect shock or stress waves coming from any potential direction as necessary.

At each interface 32, provided the adjacent acoustic impedances are different, a portion of the stress wave is transmitted and a portion of the stress wave is reflected. When the impedance of the first layer encountered is less than the impedance of the second layer encountered (I_(a)<I_(b)) a tensile stress wave is reflected. When the impedance of the first layer encountered is greater than the impedance of the second layer encountered (I_(a)>I_(b)) a compressive stress wave is reflected.

A coefficient of reflection, KR, and a coefficient of transmission, K_(T), is provided below wherein I₁ is the impedance of the first layer encountered by the stress wave and I₂ is the impedance of the layer across the interface from the first layer.

$K_{T} = \frac{2 \cdot I_{2}}{I_{2} + I_{1}}$ $K_{R} = \frac{I_{2} - I_{1}}{I_{2} + I_{1}}$

With reference to FIGS. 1-3, dissipater 20 may be constructed in various forms and positioned in different positions to protect a package 22 as desired or needed. For example, package 22 may be an electronics package 22 that is positioned proximate to explosive device 18 and therefore need a substantial amount of protection from the shock wave encountered upon detonation. Shock dissipater 20 may be positioned between package 22 and explosive device 18 for example in a sub assembly, or positioned within the housing incorporating package 22. Due to the high degree of shock, or stress, wave dissipation required dissipater 20 may be formed with more than one interface 32. A few examples of materials that may be used for layers 30 include elastomeric materials, rubber, plastic, various grades of metals, steel, iron, encapsulated water, cement and air. If more than two layers 30 are utilized, the materials of construction may be alternated, for example rubber-steel-rubber, or may include more than two materials of construction, for example rubber-steel-cement-steel. It is further noted that the same type of material may be utilized in adjacent layers, provided that the materials have dissimilar impedances. Also, the same material may be used in non-adjacent layers along the lines noted above.

From the foregoing detailed description of specific embodiments of the invention, it should be apparent that a system for dissipating impulse stress energy propagated from an explosive device to shield a device from the stress energy that is novel has been disclosed. Although specific embodiments of the invention have been disclosed herein in some detail, this has been done solely for the purposes of describing various features and aspects of the invention, and is not intended to be limiting with respect to the scope of the invention. It is contemplated that various substitutions, alterations, and/or modifications, including but not limited to those implementation variations which may have been suggested herein, may be made to the disclosed embodiments without departing from the spirit and scope of the invention as defined by the appended claims which follow. 

1. An apparatus for dampening a stress wave, the apparatus comprising a first interface formed between a first layer of material and a second layer of material, the first and second layers of material having dissimilar acoustic impedances.
 2. The apparatus of the claim 1, further including a third layer of material positioned in contact with the second layer of material to form a second interface, the second and third layers of material having dissimilar acoustic impedances.
 3. The apparatus of claim 2, wherein the acoustic impedance of the first layer of material and the acoustic impedance of the third layer of material are dissimilar.
 4. The apparatus of claim 2, wherein the acoustic impedance of the first layer is less than the acoustic impedance of the second impedance layer and the second acoustic impedance is greater than the acoustic impedance of the third layer of material.
 5. The apparatus of claim 4, wherein the acoustic impedance of the first layer of material and the acoustic impedance of the third layer of material are dissimilar.
 6. The apparatus of claim 2, wherein the acoustic impedance of the first layer is greater than the acoustic impedance of the second impedance and the second acoustic impedance is greater than the acoustic impedance of the third layer of material.
 7. A wellbore tool string, the tool string comprising: an explosive device, a package; and a shock dissipater positioned between the explosive device and the package, the shock dissipater including a first interface formed between a first layer of material and a second layer of material, the first and second layers of material having dissimilar acoustic impedances.
 8. The tool string of claim 7, wherein the shock dissipater is a sub assembly.
 9. The tool string of claim 7, further including a housing having an axial bore and a longitudinal axis, wherein the shock dissipater is positioned across the axial bore such that the first interface is oriented substantially perpendicular to the longitudinal axis.
 10. The tool string of claim 7, further including a conduit formed through the shock dissipater, the conduit being oriented substantially parallel to the longitudinal axis of the housing.
 11. The tool string of claim 7, further including a third layer of material positioned in contact with the second layer of material to form a second interface, the second and third layers of material having dissimilar acoustic impedances.
 12. The tool string of claim 11, wherein one of the layers of material is a metal, another one of the layers of material is an elastomeric material, and another one of the layers of material is foam.
 13. The tool string of claim 11, wherein the acoustic impedance of the first layer of material and the acoustic impedance of the third layer of material are dissimilar.
 14. The tool string of claim 11, wherein the acoustic impedance of the first layer is less than the acoustic impedance of the second impedance and the second acoustic impedance is greater than the acoustic impedance of the third layer of material.
 15. The tool string of claim 14, wherein the acoustic impedance of the first layer of material and the acoustic impedance of the third layer of material are dissimilar.
 16. The tool string of claim 11, wherein the acoustic impedance of the first layer is greater than the acoustic impedance of the second impedance and the second acoustic impedance is greater than the acoustic impedance of the third layer of material.
 17. The tool string of claim 11, further including a housing having an axial bore and a longitudinal axis, wherein the shock dissipater is positioned across the axial bore such that the first interface is oriented substantially perpendicular to the longitudinal axis.
 18. The tool string of claim 17, further including a conduit formed through the shock dissipater, the conduit being oriented substantially parallel to the longitudinal axis of the housing.
 19. A method of dampening stress waves propagated from the detonation of an explosive device in a wellbore to protect a device positioned downhole, the method comprising the steps of: positioning a shock dissipater between an explosive device and a package; detonating the explosive device propagating a stress wave toward the shock dissipater and the package; and reflecting a portion of the stress wave in the shock dissipater.
 20. The method of claim 19, wherein the step of reflecting includes: reflecting a tensile wave portion of the stress wave at one interface between layers of material; and reflecting a compressive wave portion of the stress wave at another interface between layers of material.
 21. The apparatus of claim 2, wherein the acoustic impedance of the first layer of material and the acoustic impedance of the third layer of material are similar. 